So 2 things im thinking. 1: If your tank is 3 weeks old you dont need a group/carbon reactor at this point. A huge dose of gfo/carbon can cause a bacterial bloom as well.
I'd shut it off until your tank is stable and you get consistent parameters and you can see if you even need it for your tank. Do you have a UV sterilizer? Run that. If its bacterial it will clear it in a day. Bacterial blooms are also common with cycling. If no Uv, do a water change.
2. Did you rinse the sand and rock well with hose etc been putting in? It may all just be dust and sediment if you didnt. So, depending on your tank size, you can take everything out, rise it well, and start over. Or you could turn your flow way down for a day and see if it settles and try vacuuming it out w a gravel vac. Do you use any filter socks? The other thing you could try is getting a hose, putting a filter sock on the end, and just siphoning straight into the sump (assuming you have one?) If it's all fine sediment suspending in the water column, maybe the filter sock can filter it without you having to pull everything. If it doesnt settle after a day with no flow- I'd go back to option 1 and treat for bacterial bloom.